A spark plug is mounted on, for example, an internal combustion engine (an engine). In this case, the spark plug is used to ignite an air-fuel mixture in a combustion chamber. Generally, the spark plug includes an insulator having an axial hole, a center electrode, a tubular metal shell main body, and a ground electrode. The center electrode is inserted into the tip end side of the axial hole of the insulator. The metal shell main body is provided on the outer periphery of the insulator. The ground electrode is joined to the metal shell main body. A gap between the ground electrode and the center electrode is a spark discharge gap.
In order to improve the corrosion resistance of the metal shell main body, a nickel layer can be provided on the surface of the metal shell main body. The nickel layer includes a metal having nickel as a principal component. A method for providing a nickel layer is disclosed in, for example, JP-A-2002-184552. In the method, a metal shell main body is immersed in a predetermined plating aqueous solution. After immersion, the metal shell main body is energized for a predetermined time period.